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Nonmotile spore-formers
Heterotrophs with flagella
Heterotrophs with restricted mobility
Rhizopoda: The Amoebas
Photosynthetic protists
The largest of the five general groups
of protists are primarily unicellular organisms with amoeboid forms. There
are three principle phyla: the forams
and the radiolarians have carbonate
shells and the rhizopods lack shells.
Heterotrophs with no permanent
locomotor apparatus
Heterotrophs with No
Permanent Locomotor
Apparatus
slight diarrhea to much more serious
conditions. In some tropical areas,
more than half of the population may
be infected. The spread of amoebic
dysentery can be limited by proper
sanitation and hygiene.
Actinopoda: The Radiolarians
The pseudopodia of amoeboid cells give
them truly amorphous bodies. One
group, however, have more distinct
structures. Members of the phylum
Actinopoda, often called radiolarians,
secrete glassy exoskeletons made of silica. These skeletons give the unicellular
organisms a distinct shape, exhibiting either bilateral or radial symmetry. The
shells of different species form many
elaborate and beautiful shapes and its
pseudopodia extrude outward along
spiky projections of the skeleton (figure
35.8). Microtubules support these cytoplasmic projections.
Hundreds of species of amoebas are
found throughout the world in both
fresh and salt waters. They are also
abundant in soil. Many kinds of amoebas are parasites of animals. Reproduction in amoebas occurs by fission, or
the direct division into two cells of
equal volume. Amoebas of the phylum
Rhizopoda lack cell walls, flagella,
meiosis, and any form of sexuality.
They do undergo mitosis, with a spindle apparatus that resembles that of
Foraminifera: Forams
other eukaryotes.
Members of the phylum Foraminifera
Amoebas move from place to place
are heterotrophic marine protists. They
by means of their pseudopods, from
range in diameter from about 20 mithe Greek words for “false” and “foot”
crometers to several centimeters. Char(figure 35.7). Pseudopods are flowing
acteristic of the group are pore-studded
projections of cytoplasm that extend
shells (called tests) composed of organic
and pull the amoeba forward or engulf
materials usually reinforced with grains
food particles, a process called cytoof inorganic matter. These grains may
plasmic streaming. An amoeba puts a FIGURE 35.7
be calcium carbonate, sand, or even
pseudopod forward and then flows into Amoeba proteus. This relatively large
plates from the shells of echinoderms or
it. Microfilaments of actin and myosin amoeba is commonly used in teaching and
spicules (minute needles of calcium
similar to those found in muscles are for research in cell biology. The projections
carbonate) from sponge skeletons.
associated with these movements. The are pseudopods; an amoeba moves by
Depending on the building materials
pseudopodia can form at any point on flowing into them. The nucleus of the
they use, foraminifera—often inforthe cell body so that it can move in any amoeba is plainly visible.
mally called “forams”—may have
direction.
shells of very different appearance.
Some kinds of amoebas form resisSome of them are brilliantly colored
tant cysts. In parasitic species such as
red, salmon, or yellow-brown.
Entamoeba histolytica, which causes
Most foraminifera live in sand or are attached to other
amoebic dysentery, cysts enable the amoebas to resist diorganisms, but two families consist of free-floating plankgestion by their animal hosts. Mitotic division takes place
tonic organisms. Their tests may be single-chambered but
within the cysts, which ultimately rupture and release
more often are multichambered, and they sometimes have a
four, eight, or even more amoebas within the digestive
spiral shape resembling that of a tiny snail. Thin cytoplastracts of their host animals. The primary infection takes
mic projections called podia emerge through openings in
place in the intestine, but it often moves into the liver and
the tests (figure 35.9). Podia are used for swimming, gathother parts of the body. The cysts are dispersed in the
ering materials for the tests, and feeding. Forams eat a wide
feces and may be transmitted from person to person in invariety of small organisms.
fected food or water, or by flies. It is estimated that up to
The life cycles of foraminifera are extremely complex,
10 million people in the United States have infections of
involving an alternation between haploid and diploid genparasitic amoebas, and some 2 million show symptoms of
erations (sporic meiosis). Forams have contributed massive
the disease, ranging from abdominal discomfort with
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FIGURE 35.9
A representative of the Foraminifera (90×). A living foram with
podia, thin cytoplasmic projections that extend through pores in
the calcareous test, or shell, of the organism.
FIGURE 35.8
Actinosphaerium, a protist of the phylum Actinopoda (300×).
This amoeba-like radiolarian has striking needlelike pseudopods.
accumulations of their tests to the fossil record for more
than 200 million years. Because of the excellent preservation of their tests and the often striking differences among
them, forams are very important as geological markers.
The pattern of occurrence of different forams is often used
as a guide in searching for oil-bearing strata. Limestones
all over the world, including the famous white cliffs of
Dover in southern England, are often rich in forams
(figure 35.10).
Amoebas, radiolarians, and forams are unicellular,
heterotrophic protists that lack cell walls, flagella,
meiosis, and sexuality. Amoebas move from place to
place by means of extensions called pseudopodia. The
pore-studded tests, or shells, of the forams have
openings through which podia extend that are used for
locomotion.
FIGURE 35.10
White cliffs of Dover. The limestone that forms these cliffs is
composed almost entirely of fossil shells of protists, including
coccolithophores (a type of algae) and foraminifera.
Chapter 35 Protists
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